Head-down bedrest is an established ground-based model simulating a number of physiological responses experienced in microgravity, including the headward fluid shift, cardiovascular deconditioning, muscle atrophy and bone demineralization. It is hypothesized that sympathoadrenal response to bedrest will modulate peripheral vascular responses, as the result of either or both altered catecholamine levels or altered adrenergic receptor density/responsiveness, and these alterations will differ depending on fluid status.